Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
Plasmas can be generated in various ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into a plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave-passing window into a discharge chamber containing a gas. Microwave discharges are advantageous because they can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas.
Capacitively-coupled RF discharges and DC discharges inherently produce high energy ions and, therefore, are often used to generate plasmas for applications where the material being processed is in direct contact with the plasma. Microwave discharges produce dense, low ion energy plasmas. Microwave discharges are also useful for applications where it is desirable to generate ions at low energy and then accelerate the ions to the process surface with an applied potential.
RF inductively coupled plasmas are particularly useful for generating large area plasmas for such applications as semiconductor wafer processing. However, some RF inductively coupled plasmas are not purely inductive because the drive currents are only weakly coupled to the plasma. Consequently, RF inductively coupled plasmas are often inefficient and require the use of high voltages on the drive coils. The high voltages produce high electrostatic fields that cause high energy ion bombardment of reactor surfaces. The ion bombardment deteriorates the reactor and can contaminate the process chamber and the material being processed. The ion bombardment can also cause damage to the material being processed.
Microwave and inductively coupled plasma sources can require expensive and complex power delivery systems. These plasma sources can require precision RF or microwave power generators and complex matching networks to match the impedance of the generator to the plasma source. In addition, precision instrumentation is usually required to ascertain and control the actual power reaching the plasma.
Igniting a plasma can also require power delivery systems that are capable of providing a power large enough to cause ionization of a plasma gas. In current systems, igniting the plasma can require supplying a high electric field (e.g., breakdown field) that is sufficient to cause a gas to excite to a state where a plasma forms, which is guided by, for example, Paschen curves. For microwave plasmas, capacitively coupled plasmas, inductively coupled plasmas, and/or glow discharge plasmas, typically a high electric field (e.g., 0.1 to 10 kV/cm) is applied to a cause an initial breakdown of the gas.
Application of a high voltage to ignite a plasma can cause several difficulties, for example, arcing outside of the ignition window (e.g., the standard operating ranges for pressure, gas flow, and/or gas species for successful ignition) and/or electrical breakdown of dielectrics (e.g., punch through). Additional undesired arcing and electrical breakdown of dielectrics can cause damage to the plasma chamber and/or system parts. Damaged parts can require frequent replacement and can be expensive. Another difficulty of applying a high voltage to ignite is that typically a custom ignition design is needed for different types/shapes of plasma sources.
Current techniques for applying a high voltage to ignite a plasma include use of a high voltage spark plug or high voltage electrodes coupled to the plasma chamber. Another current technique is applying the high voltage directly to a portion of the plasma chamber itself (e.g., block ignition). In addition to the difficulties described above, each of these ignition techniques has difficulties.
Spark plugs typically have a limited lifetime due to for example, relays used in the spark plug, thus requiring frequent replacement. High voltage electrodes typically have to withstand exposure to the plasma during processing. This can cause a limited lifetime for the electrodes and/or limited material options for the electrode. Block ignition creates a potential for plasma arcing and can limit the choices for block materials/coatings.
Therefore, it is desirable to ignite a plasma without arcing, punch through, or exposing parts and/or the plasma chamber itself to high voltages, ion bombardment, radicals, and/or undesirable arcing/heat.